Helmholtz Coils
Surely, someone out there has the vector equations describing the magnetic field iof a classic Helmholtz coil. My vector equations class, being 30+ years ago, is just too dim. I keep getting the derivation wrapped around the axle. The centerline equation, H=0.716NI/r, is readily available; however, I want to analyze the interior volume and, if practical, the exterior volume. Jim Sketoe Boeing Company (314)925-4735 james.g.ske...@boeing.com
Re: GFIs, Hairdryers, and Bathtubs ... -Reply
Don't forget, the low impedance path from the black (high side of switch) to water to human to water to neutral (white wire.) It doesn't go to ground and may not even trip the GFI! hans_mellb...@non-hp-cupertino-om5.om.hp.com 09/02/97 06:33pm You forgot two major assumptions: 1) The impedance of the water and 2)the impedance of the human in water. In water, the human has orders of magnitude of lower impedance, and therefore, the majority of the current will flow through the human assuming there is a path to ground. It is highly unlikely that there is no path to ground. In most home building codes, plastic piping is not allowed due to fire codes (they melt) perhaps with the exception of waste pipe (most of them are cast iron) Recently in the bay area, a girl was electrocuted in a swimming pool due to a faulty lighting wire. Yes, the breaker triped but too late. Also, don't forget that there are many unsuspecting grounds. The faucet, dings and scrapes on the porcelain or enamel, wet wood, concrete, stucco, morter etc. Those are all better conductors than tap water. Hans __ Reply Separator _ Subject: GFIs, Hairdryers, and Bathtubs ... Author: Non-HP-dmckean (dmck...@paragon-networks.com) at HP-ColSprings,mimegw5 List-Post: emc-pstc@listserv.ieee.org Date:8/13/97 9:51 AM Recently somewhere back in the news (couple of weeks ago), two children died when one of them used a hairdryer in the tub. A discussion this accident with some lead me to a counter-intuitive result from my experience in product safety. ** Given Situation #1: 1. Person in a tub of water sitting at the opposite end of the tub than the drain. 2. The tub is ungrounded with water in it. Ungrounded meaning that the drain is plastic piping. 3. A 2-wire hair dryer (either On or OFF) is dropped into the water at the drain end of the tub. The person in the tub is neither in contact with the hairdryer, not anything that would be grounded. The hairdryer has a plastic case. Result: 1. Since there is no path to ground from the hairdryer through the person in the tub to ground, no current should flow *thru* the person in the tub. The person is in no harm. 2. Since no current is flowing through ground, the GFI won't trip. If the water allows sufficient current to flow by shorting between the HOT and NEUTRAL in the hairdryer, the breaker will trip. ** Given Situation #2: Same as situation #1 except that the drain is now a grounded metal pipe. Is there a sufficient parallel path to be lethal to the person in the tub? Seems as though the parallel path for the hairdryer is straight to the drain. Result: 1. Person is still unharmed. 2. GFI trips. ** Are my conclusions correct? Is the only time one can be electrocuted in a tub when they are in contact with a ground and holding the hairdryer? Is a person really killed in a tub with hairdryer by drowning rather than electrocution? I'm beginning to doubt that I know exactly how a person is killed in a tub with a hairdryer. Or, am I making this way more complicated than it is? Comments?
EFT testing of three phase industrial equip
The test setups I have seen for CE testing involve single phase equipments. Looking for thoughts on EFT tests of high current three-phase equipment. Do I need to run A-B, B-C, C-A, A-GND, B-GND, C-GND, ABC-GND, AB-GND, BC-GND, CA-GND? Are there other combinations which must be addressed? Who makes an EFT gen to handle 440 volts wye? jgs
We have, under development, a large piece of equipment which
We have, under development, a large piece of equipment which we plan to sell in Europe. Its size is 2.5 meters wide, 2.1 meters high, and up to 13 meters long. Its weight is hundreds, if not thousands, of kilograms. Power requirements are 3-phase 440 vac with line currents exceeding 400 amps. Obviously, the unit is a class A heavy industrial product. My request is for advice about qualifying the unit. Methods we are considering include emissions testing on-site in the manufacturing plant. Conducted emissions will be measured using a high voltage probe such as the EMCO 3701. Radiated emissions tests would be done when the ambient is low, for example Sunday between 0100 and 0300. Immunity tests are more difficult. Do you have any suggestions?
Surge currents when powering up equipment.. -Reply
While you appear to be looking at non-military applications, the U.S. MIL-STD-1399/300A paragraph 5.2.9 limits the surge current per its Figures 14 and 15. Jim Sketoe Paul Herrick 0007515...@mcimail.com 04/22/97 09:09pm -- [ From: Paul Herrick * EMC.Ver #2.3 ] -- Does anyone know of any regulatory reason to control the startup surge current to ITE units? We have measured this startup current to be up to 90A for the first half cycle of applied voltage. The unit is cord set connected and uses four switching power supplies and has a rating of 100-120/220-240V 7/3.5A. Do safety agencies (UL/CSA/TUV) ever measure this start up current as part of a certification investigation? ++ Thanks, Paul Herrick 7515...@mcimail.com
magnetic field immunity -Reply
We here at McDonnell Douglas have done some testing to 120-140 gauss range. The test object was about 24 inches in diameter and 3 feet long. Had quite a problem generating the field over that volume. The 120-140 range was because of the copper heating. Glad to discuss the issues with you. Regards, jgs Terry robert.te...@nematron.com 04/18/97 01:00pm I am trying to perform some magnetic field immunity testing on an industrial PC. However, I have an unusually large field intensity requirement. I am looking for a lab that can perform IEC-100-4-8 style magnetic field immunity testing to an intensity level of 100 Gauss. Is anyone familiar with a test lab that could generate a power frequency magnetic field with 100 Gauss intensity? If so, please let me know. Thanks in advance for your time and attention. I look forward to your responses! Robert L. Terry Nematron Corporation 313 994 0591 Ext 235 313 994 8408 Fax robert.te...@nematron.com
Re: Toroid Isolation Power Transformers -Reply
The isolation aspect is limited to 60Hz isolation. The prime power is ungrounded. It will be fed through the isolation transformer whose secondary will be grounded to form a green wire ground/neutral connection. For this I don't think I need interwinding screens. However, I don't quite understand your comment supporting switch on voltage peaks. This seems to me to be counter productive - or am I under a misconception. I thought I would need a switch on zero to minimize inrush current. Thanks for the comments. I apoligize for the delay in getting back to you. jgs Chris Dupres cdup...@vgmicrotech.com 12/05/96 03:41am Hi Jim. You wrote: Looking for info on using toroid transformers for power isolation in applications up to 10 kVA. Their advantages are great - half the size, half the weight, and less costly. I understand they have higher inrush current, but I don't know how much. Looking for advice. I'm not sure about their use as IsolationTransformers, I always use E cores with separate bobbins for that. The Toroids MUST have earthed interwinding screens if used for isolation. Inrush currents can be very high, it's best to switch them with a Peak Voltage switching solid state device (Not zero voltage) if you can, or use circuit breakers with an 10:1 instant / long term trip ratio. They are much better at tranferring EMI from the secondary to the primary as well, they can't be used as an adjunct to filters as E cores can. Thats a tuppence worth.. Chris Dupres EMC Specialist. VG Microtech. cdup...@vgmicrotech.com tel +44 (0) 1825 761077 fax +44 (0) 1825 768343 'Opinions expressed are personal, not necessarily Corporate'
